BPDAs are attracting attention as monomers for use in producing polyimides, which are heat-resistant resins. Conventionally known methods for synthesizing BPDA include, for example, a process in which a phthalic diester, e.g., dimethyl o-phthalate, is subjected to a dimerization reaction in the presence of both a palladium catalyst and a molecular oxygen to produce a tetramethyl biphenyltetracarboxylate (as described, e.g., in JP-B-62-17564), and this compound is hydrolyzed into a biphenyltetracarboxylic acid, which is then dehydrated into a BPDA (as described, e.g., in JP-A-1-104063 and U.S. Pat. No. 4,958,002)). (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application," respectively.)
Also known is a process in which a halogenated phthalic acid, e.g., 4-chlorophthalic acid obtained by chlorinating phthalic acid, is subjected to a dimerization reaction in an aqueous solution containing an alkali as a halogen acceptor in the presence of a reducing agent and a specific palladium catalyst, thereby to produce a biphenyltetracarboxylic acid, which is then dehydrated to synthesize a BPDA (as described, e.g., in JP-A-2-53742).
However, these processes are disadvantageous in that they necessitate many reaction steps and complicated treating procedures before the desired BPDA is obtained. Therefore, it is extremely troublesome to practice the processes industrially, and the overall yield of the desired BPDA is not always high.
For these reasons, there has been a demand for a synthesizing method of a BPDA that is industrially advantageous and in that the number of reaction steps required is small. For example, if a BPDA can be produced directly from phthalic anhydride by dimerization, the treating procedure can be greatly simplified. However, there have so far been no reports concerning the direct dimerization of phthalic anhydride. It has been thought that this may be because the anhydride bond in phthalic anhydride reacts with catalysts to inhibit the dimerization reaction.